CHAP. IV.] 



THE TELESCOPE. 



259 



they can be observed at the same time, and measures of the object 

 can be taken. . 



The long focus instruments in astronomical observatories are so 

 heavy that their manipulation would be difficult unless special 



FIG. 108. Perspective view of the transit circle at Greenwich. 



arrangements were adopted. A complicated and carefully-balanced 

 mounting, called the equatorial mounting (see Plate XI.), is therefore 

 used, and by means of wheel-work and driving-clocks all the necessary 



s 2 



